Peroxidase Activity Of Respiratory Proteins. The Role Of Protein Bound Free Radicals

Abstract

Peroxidase activity of respiratory haem proteins is associated with formation of two active species on the protein when it reacts with peroxides: the oxoferryl state of the haem and a protein bound free radical. Both the free radical and the oxoferryl haem are capable of oxidising a range of substrates. General principles of formation of these two species and of their further reactions will be considered. Experimental confirmation of the proposed view involves a complex analysis of diverse pathways by which electrons can be passed between different parts of the protein. Associated with this electron transfer, the process of free radical character transfer around the protein as well as between different protein molecules can be monitored by the EPR spectroscopy. If a protein contains tyrosine residues (which is often but not always the case), the chance of observing a tyrosyl radical is high. EPR data will be presented on formation, transformation and transfer of protein bound radicals in different respiratory proteins. A method will be described that allows very accurate determination of the tyrosyl radical parameters. By using the method, it is possible to determine the three principal g-values of the radical solely from an X-band EPR spectrum. (And if a high field EPR spectrum of the radical is available, and the g-values are measured directly, the other radical parameters, e.g. the hyperfine interaction constants, can be determined much more accurately.) The radicals parameters extracted from experimental EPR spectra are then compared with either some structural features of the tyrosines (phenoxyl ring rotation angle) or with the values of the parameters obtained from the DFT calculation. This allows identification of the tyrosine in the protein responsible for the observed radical.